@SceneStorage support

[GodL]

For multi-window scenarios on iPad, Apple provides @SceneStorage for scene state restoration. Currently, swift-sharing only supports @AppStorage. Could you provide an implementation of @SceneStorage?

[mbrandonw]

Hi @GodL, I don't believe it is possible for us to provide scene storage for @Shared. The whole point of scene storage is that it is tied to a scene, which is a very view-specific concept, and so it wouldn't make sense to have @Shared(.sceneStorage(…)) outside of a view, such as in an observable model.

If I am misunderstanding something or if you have ideas to make it work we'd love to hear about it.

[DandyLyons]

The whole point of scene storage is that it is tied to a scene, which is a very view-specific concept, and so it wouldn't make sense to have @shared(.sceneStorage(…)) outside of a view, such as in an observable model.

I'm not sure if this statement is entirely correct. It is correct in the sense that a Scene contains child views and is a part of the SwiftUI/UIKit view tree. But the Scene can also represent a unique running instance of the app. For example, I can run the same app side by side in split screen mode on iPad and these will be two different Scenes. I think that state persistence of per-scene data could be relevant even outside of a view.

Unfortunately, this is all probably moot. According to the docs:

The system manages the saving and restoring of SceneStorage on your behalf. The underlying data that backs SceneStorage is not available to you, so you must access it via the SceneStorage property wrapper. The system makes no guarantees as to when and how often the data will be persisted.

So I'm not sure it would be possible for us to implement a SceneStorage persistence strategy anyways.

[mbrandonw]

But the Scene can also represent a unique running instance of the app. For example, I can run the same app side by side in split screen mode on iPad and these will be two different Scenes. I think that state persistence of per-scene data could be relevant even outside of a view.

The only way SwiftUI expresses this is via the Scene protocol, which can only be used directly in the var body: some Scene of the app entry point. The reason that SwiftUI hides the underlying storage of @SceneStorage is because they can't possibly expose the storage outside the context of a view. Only @AppStorage can do that, because it's powered by a global UserDefaults under the hood and not tied to a view.

They could perhaps have the concept of a "scene ID" that can be used to look up storage, but that may also be a very limiting API.